[Haskell-cafe] Proper exception handling

Thomas DuBuisson thomas.dubuisson at gmail.com
Sun Feb 10 14:49:39 EST 2008

Fact 1: ghc{,i} does not crash when executing this code.
Fact 2: I do not want this to crash.
Question: Is there some theoretical window between the 'catchDyn' exception handling and the recursive call to 'catchThatDamnError' that could result in an unhandled exception? (of type 'DynError', of coarse)

I suppose I am looking for an answer to this question from a language standpoint as well as a compiler pov.

As an aside: I see at least one way to be certain of the safty by wrapping the call to forkIO in 'catchDyn', reforking if an exception is caught, and passing the new ThreadId to throwConstantly via shared mutable state - I'd like to avoid all this if my current example is safe.


> import Control.Exception (catchDyn, throwDynTo)
> import Control.Concurrent (forkIO, threadDelay)
> import Control.Monad (forever)
> import Data.Dynamic
> main = do
>     tid <- forkIO catchThatDamnError
>     forever $ throwConstantly tid
> catchThatDamnError = catchDyn start (\DynError -> catchThatDamnError)
> start = do
>     threadDelay 5000
>     start
> throwConstantly tid = do
>     throwDynTo tid DynError 
> data DynError = DynError deriving (Eq, Ord, Show, Typeable)

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